I don't think you need to check AC circuitry especially since you admit to somewhat limited electrical knowledge.
If you have a button for discharge, see i you can get to it's wires. Using a voltmeter set to DC connect one lead to a good ground and then touch the other lead each of the two wires on the push button, one at a time.You should see 12 volts or so on one of the leads but not the other. The one that has no voltage should have it when you press the button.
If neither have juice you have to work back towards the fuse panel or breaker panel.
If you do have juice at both push button leads when the button is pressed then you need to look towards the macerator wiring (near the tank?)
I like to trace voltages with one of those inexpensive test lamps that has a point like an awl and a ground wire with an alligator clip. Ni measuring just look to see if the light lights. These work well on the fuse panel too. Connect the black wire to a good ground and test both sides of each fuse holder. They should all make the light go on.
Here is a link for an assortment of test lamps but even the cheap ones form Harbor Freight will do.